1. Field of the Invention
The present invention relates to an image forming apparatus including a high voltage generating circuit that applies an oscillating voltage in which a DC voltage and an AC voltage are superimposed on each other, to a charging member disposed in contact with or dose to an image carrier, and a voltage control portion that controls a peak-to-peak voltage value Vpp of the AC voltage to a target voltage.
2. Description of the Related Art
In recent years, as a charging control apparatus to be mounted in an image forming apparatus, in view of a low voltage process in which a charging control voltage to an image carrier is reduced, reduction in amount of ozone generated in charging control, reduction in cost, and the like, a charging control apparatus that employs a contact charging system has become mainstream, in which system an image carrier surface is uniformly charged by disposing a charging member of a roller type, a blade type, or the like, in contact with or dose to the surface of an image carrier and applying an oscillating voltage in which a DC voltage and an AC voltage are superimposed on each other, to the charging member. In this system, for the oscillating voltage, not only a sine wave but also any oscillation waveform that periodically changes, such as a rectangular wave, a triangular wave, or a pulse wave, can be employed.
Japanese Laid-Open Patent Publication No. 63-149668 discloses that in a case where such a contact charging system is employed, the following charging characteristics are exhibited.
Specifically, when a peak-to-peak voltage value of an AC voltage in an oscillating voltage is raised, a charging voltage of an image carrier increases in proportion thereto; when the peak-to-peak voltage value reaches about twice a charging start voltage by a DC voltage, a charging potential is saturated, and thus even if the peak-to-peak voltage value is further raised, the charging potential does not change; in order to ensure uniformity in charging, there is a need to apply an oscillating voltage having a peak-to-peak voltage that is twice or more the charging start voltage obtained upon the DC voltage application which is determined by various characteristics and the like of the image carrier; a charging voltage obtained at the time depends on a DC component of the applied voltage; and the like.
Accordingly, an oscillating voltage in which a peak-to-peak voltage of an AC voltage is set to a voltage value greater than or equal to a value (hereinafter, referred to as an “inflection point voltage”) at which a charging potential does not change even when the peak-to-peak voltage is raised to a value greater or equal to the value thereof, needs to be applied to a charging member. Note, however, that the inflection point voltage changes with a resistance value of the charging member, use environment such as temperature and humidity, deterioration over time, and the like, and thus, normally, the peak-to-peak voltage is set to a voltage having an allowance of the order of 1.5 to 2 times a pre-confirmed inflection point voltage.
However, in an area where the peak-to-peak voltage is greater than or equal to the inflection point voltage, the amount of discharge increases due to opposite discharge, and therefore the amount of ozone generated increases. Since ozone is generated near the charging member and the image carrier, a discharge product such as nitrogen oxides (NOx) which is generated from air decomposed by ozone is likely to adhere to the image carrier. When the amount of adhesion of such a discharge product increases, due to an increase in kinetic friction resistance of the surface of the image carrier, there arises a problem such as occurrence of cleaning failure due to toner escaping through a cleaner blade or occurrence of an image flow due to leakage of charge.
In view of this, in order to suppress the cleaning failure or the image flow which occurs when an oscillating voltage in which an AC voltage is superimposed on a DC voltage is applied to the charging member, the present inventors have attempted to perform so-called calibration of a charging bias voltage in which during a period in which an image forming operation is not immediately executed, i.e., when the power to the image forming apparatus is turned on, or when the image forming apparatus returns to a normal operation mode from an energy saving mode, the peak-to-peak voltage of the AC voltage is corrected to a minimum necessary voltage value, i.e., a voltage value that is greater than or equal to an inflection point voltage and near the inflection point voltage.
However, in a case where the image forming apparatus is in a low temperature environment, there appear temperature characteristics that the resistance value of the charging member exhibits a higher value than that in a room temperature environment or a high temperature environment.
When the power to the image forming apparatus is turned on or when the image forming apparatus returns to the normal operation mode from the energy saving mode, the image forming apparatus is likely to be in such a low temperature environment, and when a printing operation is repeated thereafter, along with an increase in temperature inside the apparatus, the temperature of the charging member increases, and the resistance value of the charging member decreases accordingly.
When calibration is performed in such a low temperature environment, since the peak-to-peak voltage of the AC voltage is adjusted in accordance with the resistance value of the charging member corresponding to the temperature at that time, if the temperature of the charging member increases with the later increase in environmental temperature and the resistance value of the charging member decreases accordingly, it invites a situation where a peak-to-peak voltage which is exceptionally higher than a target, appropriate peak-to-peak voltage corresponding to the charging voltage of the image carrier is applied.
This is because an inflection point voltage obtained when the resistance value of the charging member is low is lower than an inflection point voltage obtained when the resistance value of the charging member is high.
If such a condition continues, the amount of discharge product that adheres to the image carrier increases, causing inconvenience such as cleaning failure and an image flow.